Recently, many wireless devices having IEEE 802.11 wireless interfaces have been commercialized and become available. Such a wireless device generally provides high portability, and is often used in a portable device. Most of portable devices are driven by a battery, and it is, therefore, important to reduce battery consumption and prolong the service life of the battery.
The IEEE 802.11 standard defines a power save mode specification in which transmission and reception are intermittently executed using the fact that an actual communication time is discontinuous even when a wireless interface is effective. The IEEE 802.11 standard proposes an infrastructure mode in which a communication device (station) communicates with another communication device via an access point serving as a base station, and an adhoc mode in which stations communicate with each other. In the power save mode specification, the infrastructure mode is defined in detail by the IEEE 802.11 standard, and has already been employed and used in many products. To the contrary, the power save mode specification in the adhoc mode is still indefinite, and has not been adopted in products.
A large difference between the infrastructure mode and the adhoc mode is as follows. In the infrastructure mode, an access point is always a communication partner and can not utilize the power save mode (is always in the active mode). To the contrary, in the adhoc mode, even a station serving as a communication partner may utilize the power save mode, and data must always be transmitted in consideration of the state of the partner.
When the IEEE 802.11 power save mode is used, a station can take two states: an Awake state and Sleep state. The station can transmit/receive data in the Awake state, but cannot transmit/receive any data in the Sleep state. If the station wants to change its power mode in the adhoc mode, it must notify a communication partner of the power mode after checking the partner's state. This is because, if the station notifies the communication partner of a change in power mode while the communication partner cannot receive any packet, i.e., is in the Sleep state, the communication partner cannot recognize the change in power mode. In this manner, the state of a communication partner must be managed in the adhoc mode, the mechanism is complicated, and thus the adhoc mode has not been put into practical use.
As described above, it is difficult to implement power saving in the adhoc mode. This is because a communication partner can utilize the power save mode in the adhoc mode and each station must always manage the state of a communication partner.
In the adhoc mode, the number of communication partners is not always one, and all stations which join a network are candidates. No proposal has been made to permit even a simple implementation in accordance with the load of implementation and implement power saving while considering communication with all stations.
It is also possible to notify a communication partner of a change in power mode by a beacon. In the adhoc mode, a station which notifies a communication partner of a change in mode cannot always immediately transmit a beacon, and the mode change notification cannot be issued quickly. As a result, the change in power mode delays, and it becomes difficult to reduce power consumption.